Marine seismic explorations are usually carried out by a seismic cable provided with several hydrophones being towed at a certain depth. Pressure waves can be released near the cable in several ways. This usually takes place by means of air guns. The pressure wave energy moves downwards through the substratum, but parts of the pressure waves are reflected from areas where there are acoustic impedance characteristics in the substratum. Hydrophones register the reflected pressure waves in the water and transform this information to electric signals which are received and processed on the seismic ship which tows the cable. Using this method, only reflected or converted shear to pressure energy is recorded. However, it is known that down in the substratum both pressure waves and shear waves are reflected. The shear waves do not travel in water and therefore cannot be detected by a hydrophone cable.
In U.S. Pat. No. 4,725,990, a marine shear cable is suggested, which is dragged along the seabed and which is provided with geophones. The cable is supposed to detect shear forces, but it has several disadvantages and weaknesses. During employment it will firstly be problematic classifying pressure waves which are reflected from the sea's surface. Secondly, all of the components are enclosed in a sleeve, which causes blows against the cable to travel to the geophones. Nor will the geophones achieve the desired contact with the sea bed. Furthermore, only one-component data is measured.